The application of certain protective metallic coatings to alloy surfaces, particularly of the nickel base or cobalt base type are described in such U.S. Pat. Nos. as 3,540,878--Levine, et al, 3,598,638--Levine (forms of which are sometimes referred to as CODEP coating) and 3,976,436--Chang (representative of those types of coatings sometimes referred to as the MCrAl class of coatings). In addition, use of flouride ions for cleaning or treatment of metallic surfaces or materials is described in U.S. Pat. Nos. 4,098,450--Keller, et al and 4,249,963--Young. The disclosures of all of the above identified patents are hereby incorporated by reference.
The development of advanced gas turbine engines has led to the design of certain hot section parts intended to operate under increasingly more strenuous environmental conditions, for example conditions of oxidation. It is common practice in the art to improve the oxidation resistance of the surfaces of such parts through the application of metallic coatings, for example of the type identified above. The result can be improved operating life of the coated part, which can be very expensive to replace and costly to repair.